A SENIOR Australian diplomat was killed playing a bizarre "trust game" atop a Manhattan building during a booze-fuelled party celebrating the Yes vote.

New York police said Canberra's Julian Simpson, 30, was trying to placate a friend who was upset about him frightening his wife by swinging her in his arms on the roof of his swanky Lower East Side apartment building before he plunged to his death.

Australian United Nations Diplomat, Julian Simpson, in February 2011. Picture: Lauren Waugh/Facebook

"I will prove that you can trust me," police sources said Mr Simpson told his long-time friend, James Waugh.

James Waugh, who was Julian Simpson’s brother-in-law. Picture: Facebook

As he leaned over the low metal fence encircling part of the seven-storey high roof terrace, Mr Simpson said: "Let's play the trust game".

Mr Waugh, whose sister Lauren Waugh is the wife of Mr Simpson, told police he went to catch his friend, but was unable to stop him from toppling over the edge and plunging five stories to a second-floor terrace below, shortly after 1.35am Wednesday local time.

Last night Ms Waugh posted on her private Facebook account that she was "absolutely shattered".

An NYPD spokesperson said the death was being treated as an accident and they did not suspect foul play.

The family of Mr Simpson are devastated by the incident and last night Mr Simpson's brother Lachlan had left his Canberra home to visit their parents in Victoria.

Mr Simpson, who had always wanted to work for DFAT and travel the world, was two-and-a-half years into a three-year posting to New York according to his former neighbour and longtime family friend Louisa Keranas.

"It was his dream to go and travel overseas with his work. He absolutely loved his job," she told News Corp yesterday.

"This is so sad, so soon before he was due to come back."

The building where Julian Simpson fell. Picture: Stuart Ramson

The youngest of three brothers, Mr Simpson was adored by all those who knew him, according to Mrs Keranas.

"He was just such a loving, caring, generous young man. Always going out of his way to thank us for things we did to help him out."

Mrs Keranas said the group of friends were all very sensible.

"They all liked wine and collecting wine but they never partied hard."

The apartment block at which Australian diplomat Julian Simpson fell to his death. Picture: News Corp

Prime Minister Malcolm Turnbull and Foreign Minister Julie Bishop were among those to pay tribute yesterday to Mr Simpson who was Australia's second secretary to the United Nations.

Mr Simpson and Ms Waugh were celebrating with a group of about 10 after dinner and drinks in Manhattan. They headed to the couple's apartment for a nightcap.

Police said some members of the group were trying to see the Empire State Building, which is partly visible from the roof terrace and was on Tuesday night lit with rainbow colours to celebrate Australia's embrace of same sex marriage.

Julian Simpson, far right, Australian diplomat. Picture: Supplied

While up there, Mr Simpson led Mr Waugh's wife to a higher ledge on the terrace and scared her by swinging her around. Police said the group returned to Mr Simpson's apartment, where Mr Waugh confronted Mr Simpson and said he was unhappy and suggested they "step outside".

Police said Mr Simpson then set about trying to convince Mr Waugh there was no danger, and suggested the "trust game" that led to his death.

A police source said NYPD officers interviewed all of the friends gathered in the apartment in the early hours of yesterday morning, saying they had all drunk heavily.

Foreign Affairs minister Julie Bishop said she extended her "condolences to the family and loved ones of Julian Simpson, a young Australian diplomat who has died in tragic circumstances in New York".

"Julian was a diligent, professional and highly skilled diplomat, whose support I valued, particularly during UN Leaders' Week," Ms Bishop said.

"He will be remembered as someone dedicated to the service of our nation as a member of Australia's foreign service.

"The Department of Foreign Affairs and Trade will continue to provide support to Julian's family, and to other Australian diplomats in New York who have lost a valued colleague."

Prime Minister Malcolm Turnbull said: "It is a tragedy. Our hearts go out to his family. I can't provide any more details at this stage. It's a shocking tragedy - a young life lost."

The front of 19 Clinton Street, in New York's Lower East Side, where Julian Simpson died. Picture: Stuart Ramson

Tajikistan diplomat Jonibek Hikmatov wrote on Facebook: "RIP my dear friend, Julian Simpson. Shocked and saddened. My prayers go to the Australian mission to the UN and his family."

Mr Simpson had started his career as a diplomat working in media relations. He regularly travelled overseas for work, including a recent trip to Ecuador where he was part of a 55-person delegation studying sustainable urban development.